1. Field of the Invention
This invention relates generally to the thawing of frozen food by the use of acoustic energy and, more particularly, to the rapid thawing, without localized heating, of frozen block fish using extremely low-power acoustic energy at the relaxation frequency of the frozen fish flesh.
2. Description of the Prior Art
Brody et al, Food Technology, February 1959, pages 109-112, report the results of using ultrasonic (20 kc, 400 kc and 1000 kc) and high sonic (10 kc) energy to speed up the thawing of frozen food; however, the advantage of using acoustic energy at the relaxation frequency of the frozen food was not recognized, and lower acoustic frequencies (below 10 kc) were not tested. Furthermore, focused energy sources with power inputs of 100 or 1000 watts were used, causing excessive local heating. Brody et al concluded that the use of ultrasonic energy to defrost frozen foods did not appear to be feasible.
Rosenberg et al, U.S. Pat. No. 3,846,565, disclose a method of using ultrasonic energy to speed up the heating to a serving temperature of 180.degree. F. of pre-cooked frozen food wrapped in a plastic packet and immersed in boiling water. An ultrasonic transducer is coupled to the water bath to provide vibratory wave energy which causes agitation of the thawed intermediate liquid layer of the food. Rosenberg et al did not recognize the advantage of using acoustic energy at the relaxation frequency of the frozen food and were interested only in raising the temperature of the frozen food to a serving temperature of approximately 180.degree. F. in a relatively short time without concern for excessive local heating; furthermore, a high-power (250 watts at 50 kc) was required.
In Fishery Market News Report No. SD-3 issued Jan. 7, 1981, by the National Marine Fisheries Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Seattle, Wash. 98109, there was stated that the application of sonic energy at 1500 Hz to blocks of frozen fish fillets substantially increases the rate of thawing of the fillets. However, the report does not disclose either that the sonic energy should be applied at the relaxation frequency or at surprisingly lower power levels, such as 6 watts or less, to avoid degrading the quality of the thawed fish by producing cavitation or localized regions of excessive heating or cooking.